U.S. patent number 6,763,718 [Application Number 10/245,617] was granted by the patent office on 2004-07-20 for micro-electro-mechanical systems ultra-sensitive accelerometer with independent sensitivity adjustment.
This patent grant is currently assigned to The United States of America as represented by the Secretary of the Navy, The United States of America as represented by the Secretary of the Navy. Invention is credited to Monti E. Aklufi, Richard L. Waters.
United States Patent |
6,763,718 |
Waters , et al. |
July 20, 2004 |
Micro-electro-mechanical systems ultra-sensitive accelerometer with
independent sensitivity adjustment
Abstract
An accelerometer is based upon the monolithic integration of a
Fabry-Perot interferometer and a p.sup.+ n silicon photosensor.
Transmission of light through a Fabry-Perot interferometer cavity
is exponentially sensitive to small displacements in a movable
mirror due to an applied accelerating force. The photosensor
converts this displacement into an electrical signal as well as
provides for additional amplification. Because the interferometer
and photosensor are monolithically integrated on a silicon
substrate, the combination is compact and has minimal parasitic
elements, thereby reducing the accelerometer's noise level and
increasing its signal-to-noise ratio (SNR). The accelerometer's
sensitivity can be user-controlled by any one or a combination of
factors: adjusting the length between the mirrors of the
Fabry-Perot cavity; adjusting the power of the light projected to
the photosensor; and pulsing the light at a selected frequency to
minimize 1/f inherent system noise in the response of the
accelerometer.
Inventors: |
Waters; Richard L. (San Diego,
CA), Aklufi; Monti E. (San Diego, CA) |
Assignee: |
The United States of America as
represented by the Secretary of the Navy (Washington,
DC)
|
Family
ID: |
32682930 |
Appl.
No.: |
10/245,617 |
Filed: |
September 17, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
892301 |
Jun 26, 2001 |
6546798 |
|
|
|
Current U.S.
Class: |
73/514.26;
356/506 |
Current CPC
Class: |
G01P
15/093 (20130101) |
Current International
Class: |
G01C
19/56 (20060101); G01P 015/08 (); G01B
011/00 () |
Field of
Search: |
;73/514.26,514.29,514.27,514.16,514.01,657 ;356/337,450,454,506
;250/227.14,227.18 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Kwok; Helen
Attorney, Agent or Firm: Lipovsky; Peter A. Kagan; Michael
A. Dunham; Celia C.
Parent Case Text
RELATED APPLICATION
This application is a continuation-in-part of commonly assigned
U.S. patent Ser. No. 09/892,301 filed Jun. 26, 2001 now U.S. Pat.
No. 6,546,798 by inventors Richard L. Waters and Monti E. Aklufi.
This related patent is incorporated by reference herein.
Claims
What is claimed is:
1. An apparatus comprising: a pair of partially transmissive,
partially reflective, surfaces wherein a first of said surfaces is
flexibly suspended adjacent and substantially parallel to a second
of said surfaces, said surfaces defining a cavity and a cavity
length therebetween; a photosensor attached to one of said surfaces
outside of said cavity; a source of light, said light for
irradiating said photosensor through said first and second surfaces
wherein said light is also partially reflected between said
surfaces; and a cavity length adjustor independent of said
photosensor for adjusting said cavity length, wherein a change in
said cavity length is sensed by a change in said light as detected
by said photosensor.
2. The apparatus of claim 1 wherein said cavity length adjustor
includes an electrode disposed to bias said first partially
transmissive, partially reflective surface towards said second
partially transmissive, partially reflective surface.
3. The apparatus of claim 1 wherein said cavity length adjustor
includes an electrode disposed to bias said first partially
transmissive, partially reflective surface away from said second
partially transmissive, partially reflective surface.
4. The apparatus of claim 2 wherein said cavity length adjustor
includes an electrode disposed to bias said first partially
transmissive, partially reflective surface away from said second
partially transmissive, partially reflective surface.
5. The apparatus of claim 2 wherein said bias is applied between
said electrode and said first partially transmissive, partially
reflective surface.
6. The apparatus of claim 2 wherein said bias is applied between
said electrode and a flexible support of said first partially
transmissive, partially reflective surface.
7. An apparatus comprising: an interferometer, including pair of
partially transmissive, partially reflective, surfaces wherein a
first of said surfaces is flexibly suspended adjacent and parallel
to a second of said surfaces, said surfaces defining an
interferometer cavity and an interferometer cavity length
therebetween; a photosensor attached to said second surface outside
of said cavity; a source of monochromatic light, said light for
irradiating said photosensor through said first and second surfaces
wherein said light is also partially reflected between said
surfaces; and a cavity length adjustor independent of said
photosensor for adjusting said cavity length, whereby a change in
said cavity length is sensed by a change in said light as detected
by said photosensor and wherein sensitivity of said apparatus is
variable by adjusting said cavity length and is variable by
activating and de-activating said light at a selected
frequency.
8. The apparatus of claim 7 wherein said interferometer and said
photosensor are monolithically integrated on a single
substrate.
9. The apparatus of claim 7 wherein said cavity length adjustor
includes an electrode disposed so that conduction therethrough
biases said first partially transmissive, partially reflective
surface towards said second partially transmissive, partially
reflective surface.
10. The apparatus of claim 7 wherein said cavity length adjustor
includes an electrode disposed so that conduction therethrough
biases said first partially transmissive, partially reflective
surface away from said second partially transmissive, partially
reflective surface.
11. The apparatus of claim 9 wherein said cavity length adjustor
includes an electrode disposed so that conduction therethrough
biases said first partially transmissive, partially reflective
surface away from said second partially transmissive, partially
reflective surface.
12. The apparatus of claim 9 wherein said photosensor is a
photodiode having a substrate and wherein said electrode shields
said first partially transmissive, partially reflective surface
from capacitive forces exerted by said substrate.
13. The apparatus of claim 9 wherein said bias is applied between
said electrode and said first partially transmissive, partially
reflective surface.
14. The apparatus of claim 7 wherein said bias is applied between
said electrode and a flexible support of said first partially
transmissive, partially reflective surface.
15. A high sensitivity micro-electromechanical optical
accelerometer comprising: a Fabry-Perot interferometer, including
pair of partially transmissive, partially reflective, surfaces
wherein a first of said surfaces is flexibly suspended adjacent and
parallel to a second of said surfaces, said surfaces defining an
interferometer cavity and an interferometer cavity length
therebetween; a proof mass attached to said flexibly suspended
first surface; a photodiode attached to said second surface outside
of said cavity; a source of variable power, fixed wavelength, laser
light, said light for irradiating said photodiode through said
first and second surfaces wherein said light is also partially
reflected between said surfaces; and an interferometer cavity
length adjustor independent of said photodiode and disposed outside
of a path defined by said light through said partially
transmissive, partially reflective surfaces for adjusting said
cavity length, whereby a change in said cavity length due to
movement of one of said surfaces with respect to the other of said
surfaces is sensed by a change in said light as detected by said
photodiode and wherein said sensitivity of said accelerometer is
variable by adjusting said cavity length, said power of said light
and by activating and de-activating said light at a selected
frequency.
16. The apparatus of claim 15 wherein said interferometer and said
photodiode are monolithically integrated on a single substrate.
17. The apparatus of claim 15 wherein said interferometer cavity
length adjustor includes an electrode disposed so that conduction
therethrough biases said first partially transmissive, partially
reflective surface towards said second partially transmissive,
partially reflective surface.
18. The apparatus of claim 15 wherein said interferometer cavity
length adjustor includes an electrode disposed so that conduction
therethrough biases said first partially transmissive, partially
reflective surface away from said second partially transmissive,
partially reflective surface.
19. The apparatus of claim 17 wherein said interferometer cavity
length adjustor includes an electrode disposed so that conduction
therethrough biases said first partially transmissive, partially
reflective surface away from said second partially transmissive,
partially reflective surface.
20. The apparatus of claim 17 wherein said photosensor is a
photodiode having a substrate and wherein said electrode shields
said first partially transmissive, partially reflective surface
from capacitive forces exerted by said substrate.
21. The apparatus of claim 17 wherein said bias is applied between
said electrode and said first partially transmissive, partially
reflective surface.
22. The apparatus of claim 17 wherein said bias is applied between
said electrode and a flexible support of said first partially
transmissive, partially reflective surface.
Description
BACKGROUND OF THE INVENTION
This invention relates generally to perceiving acceleration upon an
object. More specifically the invention relates to the devices used
for making such perceptions, known as accelerometers and
particularly an optical accelerometer created through the
technology known as micro-electro-mechanical systems or"MEMS". With
yet greater exactness, the invention relates to adjusting the
sensitivity of such an accelerometer by selectively and
independently positioning interferometers mirrors utilized in the
invention.
Micro-electro-mechanical systems use microelectronic processing
techniques wherein mechanical devices arc reduced to the scale of
microelectronics. These processing techniques enable the
integration of both mechanical and electrical components onto a
single chip, typically made of silicon. Prior to MEMS,
accelerometer components were for the most part manufactured
separately. These components were then assembled together in a
process that could easily be complex and expensive.
Current MEMS accelerometer designs have numerous advantages over
their conventional counterparts. The MEMS accelerometers are of
small size, light weight and low cost. Their sensitivity, however,
has fallen largely in the low performance regime. Because of their
relative low sensitivity and cost, current MEMS accelerometers have
been used primarily in the automobile industry as collision airbag
sensors and for other low sensitivity applications. Although the
collision airbag sensor market is significant, it is but a small
fraction of the potential market for low cost ultra-sensitive MEMS
accelerometers.
Existing MEMS accelerometer technology is based upon either
capacitive or piezo-based designs. State of the art MEMS capacitive
accelerometers measure the charge on a capacitor to detect small
movements of a proof mass attached to a spring. However, in order
to detect sub milliG (1 G=9.8 m/s.sup.2) perturbation forces with
this technique, elaborate amplification circuitry capable of
measuring on the order of nanovolt changes in potential is
necessary. For example, typical steady state capacitance values for
MEMS accelerometers are in the 100 fFarad range, where 1
f-10.sup.-15. Furthermore, a 1 G accelerating force results in a
minute change in capacitance, on the order of 100 aF where
a=10.sup.-18. This is equivalent to sensing a change of 625
electrons across the plates of a capacitor at an applied bias of 1
volt. Alternatively stated, this is commensurate with detecting the
presence/absence of approximately 1 out of every 1000 electrons. To
amplify this small change in capacitance extremely sensitive
circuitry is required to translate the capacitance variations into
a detectable voltage output signal. Even with the addition of low
noise amplification circuitry, these MEMS accelerometers do not
have the sensitivity required for many potential applications.
Piezoelectric or piezoresistive materials produce either a
potential difference or a change in resistance when an external
pressure/force is applied. This property lends itself to
accelerometer designs. A shortcoming of piezoelectric or
piezoresistive materials is that they are also pyroresistive,
meaning that they change resistance with temperature. High
sensitivity piezo-based accelerometers are therefore difficult to
maintain. In addition, the resistance or change in potential of
such accelerometers is usually extracted from a large resistor
fabricated in the material. This large resistance leads to
increased noise, e.g. resistive noise/Johnson noise. These problems
are significant for piezo-based accelerometers. More commonly used
accelerometers therefore use the capacitive method--which also
suffers from noise but not as severely.
To realize the full potential of MEMS accelerometers, a significant
improvement in sensitivity is required. Ideally, this improvement
will minimize accelerometer inherent noise. Possible applications
of such low cost, light weight, ultra-sensitive MEMS accelerometers
include bio-mechanics, seismology, condition monitoring of machines
and structures, and robotics. In addition, an ultra-sensitive MEMS
accelerometer would dramatically improve the accuracy of guidance,
navigation, and global positioning systems (GPS) that require
sensitivity not on the order of the 1 G scale but rather the on the
order of the .mu.G scale or better.
The invention has structural similarities to an optical switch and
amplifier described in the article titled: "Micromechanical
Optoelectric Switch and Amplifier (MIMOSA)" by R. Waters et al,
IEEE Journal of Selected Topics in Quantum Electronics, 5, 33
(January/February 1999) incorporated by reference herein.
SUMMARY OF THE INVENTION
The invention is an improvement upon an accelerometer based upon
the monolithic integration of a Fabry-Perot interferometer and a
p.sup.+ n silicon photodiode. The transmission of light through a
Fabry-Perot etalon is exponentially sensitive to small
displacements in the position of a movable mirror due to changes in
an applied accelerating force. The photosensor converts this
displacement to an electrical signal as well as provides for
additional amplification. Because both the Fabry-Perot modulator
and photodiode are monolithically integrated on a silicon
substrate, the combination is compact and has minimal parasitic
elements, thereby reducing the accelerometer's noise level and
increasing its signal-to-noise ratio (SNR).
The sensitivity of the invention is user-controlled based upon any
one or a combination of factors: adjusting the length between the
mirrors of the Fabry-Perot etalon; adjusting the power of the light
projected through the mirrors to the photodiode; and activating and
deactivating the light at a selected frequency to minimize 1/f
inherent system noise in the response of the accelerometer.
In the present design, the length between the mirrors of the
Fabry-Perot etalon is adjustable by a mechanism lying outside of
the optical path of the photodiode and that does not utilizing the
photodiode itself as a conductor to aid in the adjustment of the
mirrors.
The MEMS accelerometer of the invention is calculated to be capable
of producing 1 V/G without the use of amplification circuitry. It
is estimated that when amplification circuitry is used with the
novel MEMS accelerometer of the invention, it will be more than
three orders of magnitude more sensitive than present MEMS
accelerometers using amplification circuitry. This implies that the
.mu.G sensitivity needed for navigation and GPS applications is
obtainable if voltage levels on the order of 1 .mu.V are
detectable.
In opposition to prior art designs, the invention uses a light
source rather than capacitive or piezo-based techniques for sensing
acceleration. The advantages of this include use of a small
wavelength of light for accurately measuring the movement of a
suspended inertial mass and utilizing the wave nature of light for
creating an exponentially sensitive accelerometer that is more than
three orders of magnitude more sensitive than the previous art.
An object of this invention is to provide an accelerometer of high
sensitivity.
Another object of the invention is to provide an accelerometer of
high sensitivity in which inherent (1/frequency) noise is
minimized.
A further object of this invention is to provide an optical
accelerometer of high sensitivity.
Still another object of this invention is to provide an optical
accelerometer in which light power is varied to adjust the
accelerometer's sensitivity.
Still yet another object of this invention is to provide an optical
accelerometer in which light power is varied to adjust the
accelerometer's sensitivity by decreasing system inherent
noise.
Still a further object of this invention is to provide an optical
accelerometer in which light is selectively pulsed to adjust the
accelerometer's sensitivity.
Still yet a further object of this invention is to provide an
optical accelerometer that includes a Fabry-Perot etalon in which
the distance between the etalon's mirrors is adjusted to adjust the
accelerometer's sensitivity.
Yet another object of the invention is to provide an optical
accelerometer that includes a Fabry-Perot etalon in which the
distance between the etalon's mirrors is adjusted to adjust the
accelerometer's sensitivity and in which this adjustment is done by
a mechanism lying outside the light path of the accelerometer and
that does not employ the light sensing mechanism to effectuate the
mirror adjustment.
Yet still a further object of this invention is to provide an
optical accelerometer of high sensitivity that is fabricated
through micro-electro-mechanical system (MEMS) processing.
Other objects, advantages and new features of the invention will
become apparent from the following detailed description of the
invention when considered in conjunction with the accompanied
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a side-section view of an accelerometer according to one
embodiment of the invention described by the inventors in their
above-cited patent.
FIG. 2 illustrates an exemplary top view of an embodiment of the
invention described by the inventors in their above-cited
patent.
FIG. 3 illustrates an exemplary top view of another embodiment of
the invention described by the inventors in their above-cited
patent.
FIG. 4 illustrates an exemplary top view of yet another embodiment
of the invention described by the inventors in their above-cited
patent.
FIG. 5 describes graphically the relationship between light
transmission and interferometer gap distance as described by the
inventors in their above-cited patent.
FIG. 6 shows a side-section view of the accelerometer of FIG. 1
modified by a weight added to its upper mirror as described by the
inventors in their above-cited patent.
FIG. 7 shows a generalized wiring diagram used in conjunction with
the embodiment of the invention of FIG. 1 as described by the
inventors in their above-cited patent.
FIG. 8 depicts yet another embodiment of the invention as described
by the inventors in their above-cited patent.
FIG. 9 shows a further embodiment of the invention as described by
the inventors in their above-cited patent.
FIG. 10 illustrates another embodiment of the invention
incorporating a light emitting diode as described by the inventors
in their above-cited patent.
FIG. 11 is like FIG. 1 but illustrates placement of a mirror
conductor according to one embodiment of the present invention.
FIG. 12 illustrates one embodiment of a mirror conductor as may be
used with the present invention.
FIG. 13 illustrates one embodiment of a mirror conductor as may be
used with the present invention.
FIG. 14 is like FIG. 7 but illustrates placement of a mirror
conductor and its generalized wiring diagram according to an
embodiment of the present invention.
FIG. 15 is like FIG. 6 but illustrates placement of a mirror
conductor according to another embodiment of the present
invention.
FIG. 16 is like FIG. 8 but illustrates placement of a mirror
conductor according to another embodiment of the present
invention.
FIG. 17 is like FIG. 10 but illustrates placement of a mirror
conductor according to another embodiment of the present
invention.
FIG. 18 illustrates placement of a mirror conductor according to
another embodiment of the present invention.
FIG. 19 is like FIG. 16 but further illustrates placement of a
second mirror conductor and its generalized wiring diagram
according to an embodiment of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to FIG. 1, a micro-electro-mechanical system
ultra-sensitive accelerometer (MEMSUSA) 10 as described by the
inventors in the above-cited patent is shown by way of example.
Accelerometer 10 lends itself to being made according to
well-understood steps familiar to the semiconductor processing
field and the MEMS world. Further description of this processing,
therefore, will not be described here.
Accelerometer 10 utilizes a monochromatic light source 12 such as a
fixed wavelength solid state laser, a light emitting diode or a
vertical cavity surface emitting laser (VCSEL). This light is
coupled directly or indirectly, such as via fiber-optic cable, to
an interferometer 14. Interferometer 14, in this example, is a
Fabry-Perot cavity 16.
This Fabry-Perot cavity is the optical cavity between upper and
lower mirrors. In this case, a first or upper mirror 18 of the
cavity is formed on the top surface of a hinged membrane 20 that is
flexibly suspended above and substantially parallel to a second or
lower mirror 22. Upper mirror 18 is designed to partially reflect
and partially transmit light from and into cavity 16, such as may
be accomplished by a thin semi-transparent mettalization on the top
surface of membrane 20. Lower mirror 22 exists on the surface of a
p.sup.+ region 24 created in substrate 26 which is for example of
silicon. Mirror 22 can be made for example by the semiconductor/air
interface or via the deposition of a thin semi-transparent metal on
the surface of region 24. Both mirrors 18 and 22 can be fabricated
through the deposition of various dielectric layers, known as a
dielectric stack, to form a dielectric mirror at a desired
wavelength. In addition upper mirror 18 can have a thin conducting
layer deposited either between the layers of the dielectric stack
or on top of the stack to form an electrode for electrostatic
actuation.
A p.sup.+ n junction 28 creates a photodiode used to absorb light
30. Substrate 26 is disposed upon an n.sup.+ substrate contact 32.
The p.sup.+ region reaches a metal contact 34 via a path not shown
but within substrate 26. Of course, photodiodes of other
configurations may be used, such as n.sup.+ p, pin and Schottky
diode, for example.
Operation of the proposed device can be understood by examining
both the transmission of light through the Fabry-Perot etalon for a
fixed mirror spacing and for a change in mirror spacing such as
will occur with an applied external force. A maximum in the
transmission of light through the Fabry-Perot cavity, with
monochromatic light incident normal to the surface of the mirrors,
is achieved if the distance between the mirrors is an integral
multiple of half wavelengths of the light. A maximum in light
transmission through the cavity implies a maximum in the
photo-generated current in the underlying photodiode of the
structure. Further, one of the two mirrors of the Fabry-Perot
interferometer is made such that it is hinged and therefore not
rigidly fixed in position. In the example shown, the upper mirror
is flexibly supported by four symmetrically located silicon dioxide
beams 36.
Two configurations of beam location for square and circular
structures arc shown in FIGS. 2 and 3, respectively. FIG. 4
illustrates multi levels of support beams. FIGS. 2-4 represent the
top view of partially metallized upper mirrors which are deposited
on a hinged membrane supported by support beams. The material used
to fabricate the membrane and support legs need not be the same
material and their size and geometry including the number of legs
may change to adjust the sensitivity of the hinged mirror to
applied external forces.
As illustrated graphically in FIG. 5, applied forces due to
acceleration or an electrostatic attraction between the mirrors
will change the effective optical path traversing the cavity length
between the mirrors causing an exponential change in the
transmission of light into the photodiode. Depending upon the
design of the structure, i.e. membrane material and thickness and
cavity length (air gap) distance, multiple peaks and valleys of the
transmission may occur as an external force is applied. Each
peak/valley in the transmission corresponds to a different range of
sensitivity due to a change in the effective spring constant. The
"circles" in this graph show airgap (cavity length) distances of
maximum sensitivity for the wavelength of light used. Thus, as will
be discussed further, an applied bias across the mirrors can be
used to tune the sensitivity range. Adjusting materials and
geometries during the fabrication process can also be used to
adjust the range of sensitivities.
Referring to FIG. 6, another embodiment of this invention is shown
in which metal weights 38 are patterned on the upper cavity mirror
to provide an additional inertial mass to this movable mirror. The
weights in this instance also serve to restrict light to only the
p-n junction region, thus decreasing the steady-state light
absorbed and therefore increasing the on/off ratio of the sensor.
Of course, one will realize that other weight material besides
metal may be used in this application. For example, wafer bonding
techniques may be used to provide one or more silicon weights as
inertial mass for the invention. Other weight materials and other
techniques of applying/attaching weight materials are also of
course possible.
FIG. 7 illustrates one possible biasing configuration in which
sensitivity of the accelerometer can be surmised. In this figure, a
constant current is applied through the photodiode while the
voltage, Vsensitivity, is adjusted to maintain a constant force on
the upper mirror, thus insuring a constant desired air gap (cavity
length d) and photodiode current. The control voltage,
Vsensitivity, is then compared to a reference voltage (Vref) and
the difference is amplified and filtered. In this simplified
configuration, the output voltage can be directly related to the
force applied perpendicularly to the upper mirror. Hence a change
in the output voltage can be used as a measure of the acceleration
force.
FIG. 8 is yet another embodiment of the invention shown in this
case with exemplary materials described. To further increase the
inertial mass for high sensitivity applications, a lower cavity
mirror including a p.sup.+ n photodiode, is flexibly supported by
cantilevered beams, 40. The upper cavity mirror in this embodiment
is rigidly supported.
Alternatively, FIG. 9 shows a top mirror made from a bulk silicon
wafer. In this alternative configuration of the accelerometer, two
wafers 42 and 44 are bonded together by heat or pressure. Wafer 42
defines an upper mirror configuration and supporting legs/springs.
These supports may be of the same composition as the upper mirror
or of a different composition. In this embodiment, backside etching
through wafer 42 is used to define an optical opening through which
monochromatic light may be illuminated onto the upper mirror. Wafer
44 defines the lower mirror configuration as well as a spacer used
to separate the two mirrors. Wafer 44 also includes a diffused
p-type region under the lower mirror. The thickness of the spacer
of wafer 44 together with the thickness of the supporting
legs/springs of wafer 42 define the effective optical cavity length
"d" of this configuration. In this embodiment, wafer 42's substrate
gives an additional inertial mass to the upper mirror thereby
increasing its displacement for a given acceleration and increasing
the sensitivity of the accelerometer.
Depending upon the design and geometry of the sensor as discussed
above, a number of methods can be used to bring light into the
structure. These methods include epoxy bonding a solid state laser
or light emitting diode to a packaged optical accelerometer
possessing a clear optical window. Another choice is to wafer bond
vertical cavity surface emitting lasers (VCSEL) such that they are
suitably positioned over the optical cavity. In addition, FIG. 10
illustrates how organic LEDs 46 can be deposited on a rigidly
attached transparent membrane. This embodiment is shown with
exemplary materials identified. Fiber optic cables can also be used
to bring light into the structure. The fiber optic cables may be
aligned to the top mirror of the membrane by any suitable alignment
techniques such as selectively etching alignment marks in the
silicon or etching precision openings in the bulk silicon to allow
the fiber optic core to align with the top mirror, etc.
In all of the embodiments, Pulse Width Modulation (PWM) of the
light used can be performed to avoid the characteristic one over
frequency noise (1/f) that plagues current accelerometers, thereby
enhancing the sensitivity of the accelerometer. Instead of having a
constant intensity light source, the source will be pulsed with
some characteristic frequency, fl, such that the electrical signal
generated by the accelerometer is at fl plus the maximum response
of the accelerometer. Since the 1/f noise decreases with frequency,
shifting the accelerometer response to a higher frequency will
reduce the noise from this source. The 1/f noise is inherent in all
accelerometers and electronic circuits, it can not be avoided by
simple mechanical means.
A unique advantage of the invention is its ability to vary
accelerometer sensitivity level by adjusting input light intensity.
In the quantum mechanical limit, photodiode noise (shot noise)
increases linearly with the square root of the input light power.
At the same time, however, the detected electrical signal in the
photodiode varies linearly with the input light power. Therefore,
the ratio of the detected signal to the noise generated within the
photodiode increases as the square root of the input power. This
increasing signal to noise ratio (SNR) with increasing optical
power allows one to adjust or increase the sensitivity of the
accelerometer by increasing the light power incident upon the
photosensor.
As previously described, the invention also lends itself to
sensitivity adjustment via changing of the distance between the
accelerometer's mirrors. This distance is easily altered by
applying a selected potential across the mirrors.
Though the invention has been described in terms of Silicon,
similar structures can be fabricated in a material system other
than Silicon, such as Indium Phosphide (InP) or Galium Arsenide
(GaAs), for example. These will also allow monolithic integration
of a light source with a photodiode and membrane structure. In
cases where applicable, upper and lower mirrors may be fabricated
on separate wafers by ionic, heat or pressure bonding.
Referring now to FIG. 11, the present invention will be described.
The present invention is an improvement upon the above-described
accelerometer design. As described above, one mechanism for
enhancing the sensitivity of the MEMS accelerometer is for the user
to adjust the cavity length (air gap) between the accelerometer's
interferometer mirrors.
In the previously described embodiment of the invention, this
adjustment is accomplished by providing a potential between the
photodiode and the flexibly suspended mirror. By generating an
electrostatic attraction between these elements, it is possible to
cause the two interferometer mirrors to converge. While this method
of accelerometer sensitivity adjustment has its merits, there are
some drawbacks that in cases will detract from accurate
accelerometer readings.
It has also been realized that under certain conditions current
absorbed in the photodiode photosensor of the invention generates a
voltage that can adversely affect accurate relative positioning of
the mirrors. It is also desirable at times to bias the photodiode
photosensor and this in turn can also adversely affect the relative
positioning of the interferometer mirrors.
In FIG. 11, a grid electrode 50 is shown in this cross-section as
located generally between the springs/support structure 52 of first
mirror 54 and photosensor 56. As can be seen in this figure,
electrode 50 is disposed outside a path 58 of light source 60.
FIGS. 12 and 13 illustrate two variations on how the grid electrode
may be patterned. As can be seen in FIG. 12, grid electrode 50
takes the shape of a concentric ring, shown in this figure further
from the viewer than mirror 54 and supports 52. A path is provided
through this ring so that photosensor 56 can be illuminated through
the central void defined by the electrode. In FIG. 13, mirror 54
and accompanying supports 52 are closest to the viewer while
electrode 50, not shown in this figure, is disposed directly
underneath supports 52. Of course, other geometrical patterns and
variations for the grid electrode exist that may allow for
increased sensitivity and/or control of the suspended mirror
displacement.
Referring now to FIG. 14, to provide selective mirror positioning,
the grid electrode may be maintained at a constant voltage and the
voltage on the flexibly suspended mirror changed, or the voltage on
the flexibly suspended mirror is kept constant and the voltage on
the grid electrode changed.
The grid electrode may be used in a force-rebalance mode to
decouple feedback from the accelerating force sensing element (in
this case flexibly suspended mirror 54). The force rebalance mode
keeps the total force on this mirror constant such that, the force
due to input acceleration plus the capacitive force exerted by the
grid electrode on mirror 54 is constant. This is achieved by
applying a variable potential to the grid electrode thus adjusting
the capacitive force as the input accelerating force changes,
resulting in a constant total force applied. This also prevents
stiction of the upper mirror to the lower mirror which has been
reported to be a common problem for the prior art.
The grid electrode also shields the upper (first) mirror from any
capacitive force exerted by the photosensor substrate. The
capacitive force from this substrate is undesirable because its
potential and hence force may change as the acceleration changes
and hence collected current in the photodiode changes. Depending
upon the point of operation this feedback can be either positive or
negative.
The grid electrode allows not only the initial cavity length
between the interferometer mirrors to be adjusted but also allows
adjustment of the effective spring constant of the mirror support
structure. The ability to independently adjust the spring constant
allows the dynamic range of the accelerometer to be adjusted.
FIGS. 15-17 show alternative arrangements of utilizing such an
electrode with many of the embodiments of the accelerometers
described above. FIG. 18 illustrates an optical sensor wherein
electrode 50 is encapsulated underneath mirror 62 and within a
region below the spring/support structure of mirror 54.
FIG. 19 shows another embodiment of the invention in which an
electrode 64 is disposed to draw mirror 54 away from mirror 62.
Electrode 64 may be effectuated via wafer bonding a suitable
substrate 66 with patterned electrodes. Use of such an "upper"
electrode is possible in conjunction with the previously described
grid electrode, as is shown in FIG. 19. One advantage of this
technique, among others, is the ability to adjust the
interferometer cavity length in both positive and negative
directions.
There are numerous advantages in using control electrodes in
conjunction with an accelerometer as described herein.
The invention provides independent control of the accelerometer's
spring constant. By varying the spring constant, the dynamic range
of a particular accelerometer sensor may be adjusted. Identical
accelerometers may be arrayed such that the spring constant for
each is adjusted. In this manner, the overall dynamic range of the
composite accelerometer array is increased over that of an
individual accelerometer sensor.
By using the invention, it is possible to decouple feedback from
the accelerometer's force sensing element, i.e. feedback control is
obtained via the control electrode as opposed to a voltage directly
on the sensing element.
The invention permits adjustment of the accelerometer's cavity
length to accommodate tuning to an optimal operating point.
The invention allows for the local adjustment of the potentials on
either side of the photosensor diode junction, e.g. the potentials
on the p+ and n regions of the photodiode can be adjusted without
affecting the net force that the force sensing element feels. This
ability to adjust the potentials across the photodiode also
increases flexibility in the design of control/interface
electronics.
In the event that an unwanted spurious external/internal force acts
upon the force-sensing element of the accelerometer, the control
electrode can be used to apply a force equal and opposite to
counter balance (or force rebalance) the system and adjust the net
force to zero. In the "force rebalance mode" described, the control
electrode helps to prevent stiction of one interferometer mirror to
the other while simultaneously decoupling the interferometer cavity
length adjusting force from the accompanying photosensor.
A further advantage of the invention is that field lines generated
by the photosensor substrate terminate on the control electrode
thereby shielding these from the upper interferometer mirror,
thereby decoupling movement of this mirror from the bias on the
substrate.
The control electrodes described arc not meant to be directed
towards a single "lower" and a single "upper" electrode. One may
incorporate a series or matrix of electrodes disposed at either
location and still fall within the spirit of the invention and
described herein.
Though the invention has been described as a sensor for sensing
acceleration, it is by no means limited to this particular
application. It can be envisioned that various other force sensing
applications of the invention are possible. Included among these,
but without limitations thereto, are sensing application in the
inertial, magnetic, pressure, electrostatic and thermal areas for
example.
In addition, the invention has been largely described by way of
example to employ a monochromatic light source. The invention is
however also considered usable with light of a plurality of colors
or wavelengths.
Obviously, many modifications and variations of the invention are
possible in light of the above description. It is therefore to be
understood that within the scope of the claims the invention may be
practiced otherwise than as has been specifically described.
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